Gasoline pumps typically post octane numbers as an average of two
different values. Often you may see the octane rating quoted as
(R+M)/2. One value is the research octane number (RON), which is
determined with a test engine running at a low speed of 600 rpm. The
other value is the motor octane number (MON), which is determined with
a test engine running at a higher speed of 900 rpm. If, for example, a
gasoline has an RON of 98 and a MON of 90, then the posted octane
number would be the average of the two values or 94.

There you have it. So in a nutshell, it seems it's a test at slightly different RPMs.

Related posts

*gasp* RegistrationIn some of my recent plumbing work to the site, I modified the state handling and decided along with...Once Again: It Was Too Thick to PourThe title here is a bit esoteric and doesn't describe the contents in this post: We recently got ba...Mix Registration OpenRegistration is now open for MIX '07 in Las Vegas.
Geared toward Web developers, designers, online a...